Process for the synthesis of 2,5-dihydroxyterephthalic acid

ABSTRACT

2,5-dihydroxyterephthalic acid is produced in high yields and high purity from 2,5-dihaloterephthalic acid by contact with a copper source and a ligand that coordinates to copper under basic conditions.

TECHNICAL FIELD

This invention relates to the manufacture of hydroxybenzoic acids, which are valuable for a variety of purposes such as use as intermediates or as monomers to make polymers.

BACKGROUND

Hydroxybenzoic acids are useful as intermediates in the manufacture of many valuable materials including pharmaceuticals and compounds active in crop protection, and are also useful as monomers in the production of polymers. In particular, 2,5-dihydroxyterephthalic acid (Formula I, “DHTA”) is a useful monomer for the synthesis of high strength fibers such as those made from poly[(1,4-dihydrodiimidazo[4,5-b:4′,5′-e]pyridine-2,6-diyl) (2,5-dihydroxy-1,4-phenylene)].

Various preparations of 2,5-dihydroxyterephthalic acid and other hydroxybenzoic acids are known. Marzin, in Journal fuer Praktische Chemie, 1933, 138, 103-106, teaches the synthesis of 2,5-dihydroxyterephthalic acid from 2,5-dibromoterephthalic acid (Formula II, “DBTA”) in the presence of copper powder.

Singh et al, in Jour. Indian Chem. Soc., Vol. 34, No. 4, pages 321-323 (1957), report the preparation of a product that includes DHTA by the condensation of DBTA with phenol in the presence of KOH and copper powder.

Rusonik et al, Dalton Transactions, 2003, 2024-2028, describe the transformation of 2-bromobenzoic acid into salicylic acid, benzoic acid, and diphenoic acid in a reaction catalyzed by Cu(I) in the presence of various ligands. A tertiary tetraamine minimizes the formation of diphenoic acid in use with Cu(I).

Comdom et al, Synthetic Communications, 32(13), 2055-59 (2002), describe a process for the synthesis of salicylic acids from 2-chlorobenzoic acids. Stoichiometric amounts of pyridine (0.5 to 2.0 moles per mole of 2-chlorobenzoic acid) are used such as at least 1.0 mole pyridine per mole 2-chlorobenzoic acid. Cu powder is used as a catalyst along with the pyridine.

The various prior art processes for making hydroxybenzoic acids are characterized by long reaction times, limited conversion resulting in significant productivity loss, or the need to run under pressure and/or at higher temperatures (typically 140 to 250° C.) to get reasonable rates and productivity. A need therefore remains for a process by which 2,5-dihydroxy terephthalic acid can be produced economically; with low inherent operational difficulty; and with high yields and high productivity in both small- and large-scale operation, and in batch and continuous operation.

SUMMARY

One embodiment of this invention provides a process for preparing 2,5-dihydroxyterephthalic acid by (a) contacting a 2,5-dihaloterephthalic acid (III)

where X=Cl, Br, or I with base in water to form therefrom the corresponding dibasic salt of 2,5-dihaloterephthalic acid; (b) contacting the dibasic salt of 2,5-dihaloterephthalic acid with base in water, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid from the dibasic salt of 2,5-dihaloterephthalic acid at a solution pH of at least about 8; (c) optionally, separating the dibasic salt of 2,5-dihydroxyterephthalic acid from the reaction mixture in which it is formed; and (d) contacting the dibasic salt of 2,5-dihydroxyterephthalic acid with acid to form therefrom 2,5-dihydroxyterephthalic acid.

Yet another embodiment of this invention provides a process for preparing a 2,5-dialkoxyterephthalic acid by preparing a 2,5-dihydroxyterephthalic acid in the manner described above and then converting the 2,5-dihydroxyterephthalic acid to a 2,5-dialkoxyterephthalic acid.

Yet another embodiment of this invention consequently provides a process for preparing 2,5-dialkoxyterephthalic acid by (a) contacting a 2,5-dihaloterephthalic acid (III)

where X=Cl, Br, or I with base in water to form therefrom the corresponding dibasic salt of 2,5-dihaloterephthalic acid; (b) contacting the dibasic salt of 2,5-dihaloterephthalic acid with base in water, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid from the dibasic salt of 2,5-dihaloterephthalic acid at a solution pH of at least about 8; (c) optionally, separating the dibasic salt of 2,5-dihydroxyterephthalic acid from the reaction mixture in which it is formed; (d) contacting the dibasic salt of 2,5-dihydroxyterephthalic acid with acid to form therefrom a 2,5-dihydroxyterephthalic acid; and (e) converting the 2,5-dihydroxyterephthalic acid to a 2,5-dialkoxyterephthalic acid.

Yet another embodiment of this invention provides a process for preparing a 2,5-dihydroxyterephthalic acid or a 2,5-dialkoxyterephthalic acid as described above that further includes a step of subjecting the 2,5-dihydroxyterephthalic acid or the 2,5-dialkoxyterephthalic acid to a reaction to prepare therefrom a compound, monomer, oligomer or polymer.

Yet another embodiment of this invention consequently provides a process for preparing a compound, monomer, oligomer or polymer by (a) contacting a 2,5-dihaloterephthalic acid (III)

where X=Cl, Br, or I with base in water to form therefrom the corresponding dibasic salt of 2,5-dihaloterephthalic acid; (b) contacting the dibasic salt of 2,5-dihaloterephthalic acid with base in water, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid from the dibasic salt of 2,5-dihaloterephthalic acid at a solution pH of at least about 8; (c) optionally, separating the dibasic salt of 2,5-dihydroxyterephthalic acid from the reaction mixture in which it is formed; (d) contacting the dibasic salt of 2,5-dihydroxyterephthalic acid with acid to form therefrom 2,5-dihydroxyterephthalic acid; (e) optionally, converting the 2,5-dihydroxyterephthalic acid to a 2,5-dialkoxyterephthalic acid; and (f) subjecting the 2,5-dihydroxyterephthalic acid and/or the 2,5-dialkoxyterephthalic acid to a reaction to prepare therefrom a compound, monomer, oligomer or polymer.

In yet another embodiment, the ligand in one or more of the processes described herein may be an amino acid in which the amine nitrogen and the carboxyl carbon are separated by no more than two carbon atoms.

DETAILED DESCRIPTION

This invention provides a high yield and high productivity process for preparing a 2,5-dihydroxyterephthalic acid by contacting a 2,5-dihaloterephthalic acid with base to form the dibasic salt of 2,5-dihaloterephthalic acid; contacting the dibasic salt of 2,5-dihaloterephthalic acid with base, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid; and then contacting the dibasic salt of 2,5-dihydroxyterephthalic acid with acid to form the 2,5-dihydroxyterephthalic acid product. The term “dibasic salt” as used herein denotes the salt of a dibasic acid, which is an acid that contains two replaceable hydrogen atoms per molecule.

Suitable dihaloterephthalic acids with which the process of this invention is started include 2,5-dibromoterephthalic acid, 2,5-dichloroterephthalic acid, and 2,5-diiodoterephthalic acid, or mixtures thereof. 2,5-dibromoterephthalic acid (“DBTA”)is commercially available. It can, however, be synthesized, for example, by oxidation of p-xylene in aqueous hydrogen bromide (McIntyre et al, Journal of the Chemical Society, Abstracts, 1961, 4082-5), by bromination of terephthalic acid or terephthaloyl chloride (U.S. Pat. No. 3,894,079), or by oxidation of 2,5-dibromo-1,4-dimethylbenzene (DE 1,812,703). 2,5-dichloroterephthalic acid is also commercially available. It can, however, be synthesized, for example, by oxidation of 2,5-dichloro-1,4-dimethylbenzene [Shcherbina et al, Zhurnal Prikladnoi Khimii (Sankt-Peterburg, Russian Federation, 1990)], 63(2), 467-70. 2,5-diiodoterephthalic acid can be synthesized, for example, by oxidation of 2,5-diiodo-1,4-dimethylbenzene [Perry et al, Macromolecules (1995), 28(10), 3509-15].

In step (a), 2,5-dihaloterephthalic acid is contacted with base in water to form therefrom the corresponding dibasic salt of 2,5-dihaloterephthalic acid. In step (b), the dibasic salt of 2,5-dihaloterephthalic acid is contacted with base in water, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid from the dibasic salt of 2,5-dihaloterephthalic acid.

The base used in step (a) and/or step (b) may be an ionic base, and may in particular be one or more of a hydroxide, carbonate, bicarbonate, phosphate or hydrogen phosphate of one or more of Li, Na, K, Mg or Ca. The base used may be water-soluble, partially water-soluble, or the solubility of the base may increase as the reaction progresses and/or as the base is consumed. NaOH and Na₂CO₃ are preferred, but other suitable organic bases may be selected, for example, from the group consisting of trialkylamines (such as tributylamine); N,N,N′,N′-tetramethylethylenediamine; and N-alkyl imidazoles (for example, N-methylimidazole). In principle any base capable of maintaining a pH above 8 and/or binding the acid produced during the reaction of the 2,5-dihaloterephthalic acid is suitable.

The specific amounts of base to be used in steps (a) and/or (b) depend on the strength of the base. In step (a), 2,5-dihaloterephthalic acid is preferably contacted with at least about two equivalents of base, preferably a water-soluble base, per equivalent of 2,5-dihaloterephthalic acid. One “equivalent” as used for a base in this context is the number of moles of base that will react with one mole of hydrogen ions; for an acid, one equivalent is the number of moles of acid that will supply one mole of hydrogen ions.

In step (b), enough base should be used to maintain a solution pH of at least about 8, or at least about 9, or at least about 10, and preferably between about 9 and about 11. Thus, typically in step (b), the dibasic salt of 2,5-dihaloterephthalic acid is contacted with at least about two equivalents of base, such as a water-soluble base, per equivalent of the dibasic salt of 2,5-dihaloterephthalic acid.

In alternative embodiments, however, it may be desirable in steps (a) and (b) to use a total of at least about 4 to about 5 equivalents of base, such as a water-soluble base, in the reaction mixture per equivalent of 2,5-dihaloterephthalic acid originally used at the start of the reaction. A base used in an amount as described above is typically a strong base, and is typically added at ambient temperature. The base used in step (b) may be the same as, or different than, the base used in step (a).

As mentioned above, in step (b), the dibasic salt of 2,5-dihaloterephthalic acid is also contacted with a copper source in the presence of a ligand that coordinates to copper. The copper source and the ligand may be added sequentially to the reaction mixture, or may be combined separately (for example, in a solution of water or acetonitrile) and added together. The copper source may be combined with the ligand in the presence of oxygen in water, or be combined with a solvent mixture containing water.

From the presence together in the reaction mixture of the copper source and the ligand, in a basic solution of the dibasic salt of the 2,5-dihaloterephthalic acid, there is obtained an aqueous mixture containing the dibasic salt of 2,5-dihydroxyterephthalic acid, copper specie(s), the ligand, and a halide salt. If desired, the dibasic salt of 2,5-dihydroxyterephthalic acid may, at this stage and before the acidification in step (d), be separated from the mixture [as optional step (c)], and may be used as a dibasic salt in another reaction or for other purposes.

The dibasic salt of 2,5-dihydroxyterephthalic acid is then contacted in step (d) with acid to convert it to the 2,5-dihydroxyterephthalic acid product. Any acid of sufficient strength to protonate the dibasic salt is suitable. Examples include without limitation hydrochloric acid, sulfuric acid and phosphoric acid.

The reaction temperature for steps (a) and (b) is preferably between about 60 and about 120° C., more preferably between about 75 and about 95° C.; and the process thus in various embodiments involves a step of heating the reaction mixture. The solution is typically allowed to cool before the acidification in step (d) is carried out. In various embodiments, oxygen may be excluded during the reaction.

The copper source is copper metal [“Cu(0)”], one or more copper compounds, or a mixture of copper metal and one or more copper compounds. The copper compound may be a Cu(I) salt, a Cu(II) salt, or mixtures thereof. Examples include without limitation CuCl, CuBr, CuI, Cu₂SO₄, CuNO₃, CuCl₂, CuBr₂, CuI₂, CuSO₄, and Cu(NO₃)₂. CuBr is preferred. The amount of copper provided is typically about 0.1 to about 5 mol % based on moles of 2,5-dihaloterephthalic acid.

When the copper source is Cu(0), Cu(0), copper bromide and a ligand may be combined in the presence of air. In the case of Cu(0) or Cu(I), a predetermined amount of metal and ligand may be combined in water, and the resulting mixture may be reacted with air or dilute oxygen until a colored solution is formed. The resulting metal/ligand solution is added to the reaction mixture containing the dibasic salt of 2,5-dihaloterephthalic acid and base in water.

The ligand may be an amino acid in which the amine nitrogen and the carboxyl carbon are separated by no more than two carbon atoms, for example, α-amino acids, in which the amine N is attached to a carbon atom that is next to the carboxyl group. Examples of suitable amino acid ligands include without limitation:

A ligand suitable for use herein may be selected as any one or more or all of the members of the whole population of ligands described by name or structure above. A suitable ligand may, however, also be selected as any one or more or all of the members of a subgroup of the whole population, where the subgroup may be any size, and where the subgroup is formed by omitting any one or more of the members of the whole population as described above. As a result, the ligand may in such instance not only be selected as one or more or all of the members of any subgroup of any size that may be formed from the whole population of ligands as described above, but the ligand may also be selected in the absence of the members that have been omitted from the whole population to form the subgroup.

In various embodiments, the ligand may be provided in an amount of about 1 to about 10, preferably about 1 to about 2, molar equivalents of ligand per mole of copper. As used herein, the term “molar equivalent” indicates the number of moles of ligand that will interact with one mole of copper.

When the 2,5-dihaloterephthalic acid is 2,5-dibromoterephthalic acid, the copper source may be Cu(0) and/or a Cu(I) salt, and it may be combined with the ligand in the presence of oxygen in water, or a solvent mixture containing water. Alternatively, when the Cu(I) salt is CuBr, and the ligand is valine, proline, or anthranilic acid, the ligand may be provided in an amount of two molar equivalents per mole of copper, and the CuBr may be combined with the ligand in the presence of water and air.

The ligand is believed to facilitate the action of the copper source as a catalyst, and/or the copper source and the ligand are believed to function together to act as a catalyst, to improve one or more attributes of the reaction.

The process described above also allows for effective and efficient synthesis of related compounds, such as a 2,5-dialkoxy terephthalic acid, which may be described generally by the structure of Formula V1:

wherein R⁹ and R¹⁰ are each independently a substituted or unsubstituted C¹⁻¹⁰ alkyl group. R⁹ and R¹⁰ are, when unsubstituted, univalent groups containing only carbon and hydrogen. In any of those alkyl groups, however, one or more O or S atoms may optionally be substituted for any one or more of the in-chain carbon atoms, provided that the resulting structure contains no —O—O— or —S—S— moieties, and provided that no carbon atom is bonded to more than one heteroatom.

A 2,5-dihydroxy terephthalic acid, as prepared by the process of this invention, may be converted to a 2,5-dialkoxy terephthalic acid, and such conversion may be accomplished, for example, by contacting a 2,5-dihydroxy terephthalic acid under basic conditions with a dialkyl sulfate of the formula R⁹, R¹⁰ SO₄. One suitable method of running such a conversion reaction is as described in Austrian Patent No. 265,244. Suitable basic conditions for such conversion are a solution pH of at least about 8, or at least about 9, or at least about 10, and preferably about 9 to about 11, using one or more bases such as described above.

In certain embodiments, it may be desired to separate the 2,5-dihydroxyterephthalic acid from the reaction mixture in which it was formed before converting it to a 2,5-dialkoxyterephthalic acid.

The process described above also allows for effective and efficient synthesis of products made from the resulting 2,5-dihydroxyterephthalic acid or 2,5-dialkoxyterephthalic acid such as a compound, a monomer, or an oligomer or polymer thereof. These produced materials may have one or more of ester functionality, ether functionality, amide functionality, imide functionality, imidazole functionality, carbonate functionality, acrylate functionality, epoxide functionality, urethane functionality, acetal functionality, and anhydride functionality.

Representative reactions involving a material made by the process of this invention, or a derivative of such material, include, for example, making a polyester from a 2,5-dihydroxyterephthalic acid and either diethylene glycol or triethylene glycol in the presence of 0.1% of ZN₃(BO₃)₂ in 1-methylnaphthalene under nitrogen, as disclosed in U.S. Pat. No. 3,047,536 (which is incorporated in its entirety as a part hereof for all purposes). Similarly, a 2,5-dihydroxyterephthalic acid is disclosed as suitable for copolymeriztion with a dibasic acid and a glycol to prepare a heat-stabilized polyester in U.S. Pat. No. 3,227,680 (which is incorporated in its entirety as a part hereof for all purposes), wherein representative conditions involve forming a prepolymer in the presence of titanium tetraisopropoxide in butanol at 200-250° C., followed by solid-phase polymerization at 280° C. at a pressure of 0.08 mm Hg.

A 2,5-dihydroxyterephthalic acid has also been polymerized with the trihydrochloride-monohydrate of tetraminopyridine in strong polyphosphoric acid under slow heating above 100° C. up to about 180° C. under reduced presuure, followed by precipitation in water, as disclosed in U.S. Pat. No. 5,674,969 (which is incorporated in its entirety as a part hereof for all purposes); or by mixing the monomers at a temperature from about 50° C. to about 110° C., and then 145° C. to form an oligomer, and then reacting the oligomer at a temperature of about 160° C. to about 250° C. as disclosed in U.S. Provisional Application No. 60/665,737, filed Mar. 28, 2005 (which is incorporated in its entirety as a part hereof for all purposes), published as WO 2006/104974. The polymer that may be so produced may be a pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene) polymer such as a poly(1,4-(2,5-dihydroxy)phenylene-2,6-pyrido[2,3-d: 5,6-d′]bisimidazole) polymer, or a poly[(1,4-dihydrodiimidazo[4,5-b:4′,5′-e]pyridine-2,6-diyl) (2,5-dihydroxy-1,4-phenylene)]polymer. The pyridobisimidazole portion thereof may, however, be replaced by any or more of a benzobisimidazole, benzobisthiazole, benzobisoxazole, pyridobisthiazole and a pyridobisoxazole; and the 2,5-dihydroxy-p-phenylene portion thereof may be replace the derivative of one or more of isophthalic acid, terephthalic acid, 2,5-pyridine dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 4,4,-diphenyl dicarboxylic acid, 2,6-quinoline dicarboxylic acid, and 2,6-bis(4-carboxyphenyl)pyridobisimidazole.

EXAMPLES

This invention is further defined in the following examples. These examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and do not limit the scope of the appended claims. From the above discussion and these examples, the essential characteristics of this invention may be ascertained, and, without departing from the spirit and scope thereof, modifications of the invention may be made to adapt it to various uses and conditions.

Materials. The following materials were used in the examples. All reagents were used as received. Product purity was determined by ¹H NMR.

Proline (99% purity) was obtained from Aldrich Chemical Company (Milwaukee, Wis., USA). The 2,5-dibromoterephthalic acid (95+% purity) was obtained from Maybridge Chemical Company Ltd.(Cornwall, United Kingdom).

Copper powder (“Cu”) (99.5% purity) was obtained from Alfa Aesar (Ward Hill, Massachusetts, USA). Copper(I) bromide (“CuBr”) (98% purity) was obtained from Acros Organics (Geel, Belgium). Copper(II) bromide (“CuBr₂”) (99% purity) was obtained from Aldrich Chemical Company (Milwaukee, Wis., USA).

Na₂CO₃ (99.5% purity) was obtained from EM Science (Gibbstown, New Jersey, USA).

The term “35% aqueous HCl” as used below denotes aqueous hydrochloric acid whose concentration is 35 grams of HCl per 100 mL of solution. The terms “H₂O” and “water” refer to distilled water.

As used herein, the term “conversion” denotes to how much reactant was used up as a fraction or percentage of the theoretical amount. As used herein, the term “selectivity” for a product P denotes the molar fraction or molar percentage of P in the final product mix. The conversion times the selectivity thus equals the maximum “yield” of P; the actual or “net” yield will normally be somewhat less than this because of sample losses incurred in the course of activities such as isolating, handling, drying, and the like. As used herein, the term “purity” denotes what percentage of the in-hand, isolated sample is actually the specified substance.

The meaning of abbreviations is as follows:, “CONV” means conversion, “h” means hour(s), “mL” means milliliter(s), “mmol” means millimole(s), “NMR” means nuclear magnetic resonance spectroscopy, “SEL” means selectivity, “T” means temperature, and “t” means time.

Examples 1 Examples 2-4 Comparative

Under nitrogen 10 mmol of 2,5-dibromoterephthalic acid was stirred with a solution of 25 mmol Na₂CO₃ in 10 mL H₂O at 50-75° C. until all of the 2,5-dibromoterephthalic acid was dissolved. Separately, 0.01 mmol of the copper compound [Example 1 and Examples 2 and 3 (Comparative)] or 0.05 mmol of Cu powder [Example 4 (Comparative)], respectively, was mixed with 1 mL of deionized water and 0.02 mmol of the ligand (Examples 1-3) or no ligand [Examples 2-4 (Comparative)], respectively, and the resulting copper containing mixture was added to the reaction mixture containing 2,5-dibromoterephthalic acid and Na₂CO₃ in H₂O. The resulting reaction mixture was heated at the given temperature and for the given time (Table 1). After cooling to ambient temperature, the reaction mixtures were carefully acidified with 35% aqueous HCl. The products were isolated by filtration, washed with water and dried under vacuum. The crude reaction product was analyzed by ¹H NMR (d6-dmso). The results are summarized in Table 1.

TABLE 1 CONV SEL T Cu Ligand Example Ligand (% mol) (% mol) (° C.) t (h) source Structure 1 Proline 100 91 80 3 CuBr₂

2 (comp.) None 35 10 80 3 CuBr None 3 (comp.) None 0 0 80 3 CuBr₂ None 4 (comp.) None 32 9 80 3 Cu None

Where an embodiment of this invention is stated or described as comprising, including, containing, having, being composed of or being constituted by certain features, it is to be understood, unless the statement or description explicitly provides to the contrary, that one or more features in addition to those explicitly stated or described may be present in the embodiment. An alternative embodiment of this invention, however, may be stated or described as consisting essentially of certain features, in which embodiment features that would materially alter the principle of operation or the distinguishing characteristics of the embodiment are not present therein. A further alternative embodiment of this invention may be stated or described as consisting of certain features, in which embodiment, or in insubstantial variations thereof, only the features specifically stated or described are present.

Where the indefinite article “a” or “an” is used with respect to a statement or description of the presence of a step in a process of this invention, it is to be understood, unless the statement or description explicitly provides to the contrary, that the use of such indefinite article does not limit the presence of the step in the process to one in number.

Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. 

1. A process for preparing 2,5-dihydroxyterephthalic acid comprising the steps of: (a) contacting a 2,5-dihaloterephthalic acid, as described generally by Formula III

wherein X=Cl, Br, or I, with base in water to form therefrom the corresponding dibasic salt of the 2,5-dihaloterephthalic acid; (b) contacting the dibasic salt of the 2,5-dihaloterephthalic acid with base in water, and with a copper source in the presence of a ligand that coordinates to copper, to form the dibasic salt of 2,5-dihydroxyterephthalic acid from the dibasic salt of 2,5-dihaloterephthalic acid at a solution pH of at least about 8; wherein the ligand comprises an amino acid in which the amine nitrogen and the carboxyl carbon are separated by no more than two carbon atoms; (c) optionally, separating the dibasic salt of 2,5-dihydroxyterephthalic acid from the reaction mixture in which it is formed; and (d) contacting the dibasic salt of 2,5-dihydroxyterephthalic acid with acid to form therefrom 2,5-dihydroxyterephthalic acid.
 2. A process according to claim 1 wherein, in step (a), the 2,5-dihaloterephthalic acid is contacted with at least about two normal equivalents of water-soluble base per equivalent of the 2,5-dihaloterephthalic acid.
 3. A process according to claim 1 wherein, in step (b), the dibasic salt of the 2,5-dihaloterephthalic acid is contacted with at least about two normal equivalents of water-soluble base per equivalent of the dibasic salt of the 2,5-dihaloterephthalic acid.
 4. A process according to claim 1 wherein, in steps (a) and (b), a total of about 4 to about 5 normal equivalents of water-soluble base are added to the reaction mixture per equivalent of 2,5-dihaloterephthalic acid.
 5. A process according to claim 1 wherein the copper source comprises Cu(O), a Cu(I) salt, a Cu(II) salt, or a mixture thereof.
 6. A process according to claim 1 wherein the copper source is selected from the group consisting of CuCl, CuBr, CuI, Cu₂SO₄, CuNO₃, CUCl₂, CuBr₂, CUI₂, CuSO₄, Cu (NO₃)₂, and mixtures thereof.
 7. A process according to claim 1 wherein the ligand is an α-amino acid.
 8. A process according to claim 1 wherein the ligand is valine, proline, or N-methylanthranilic acid.
 9. A process according to claim 1 further comprising a step of combining the copper source with the ligand before adding them to the reaction mixture.
 10. A process according to claim 1 wherein the copper source comprises CuBr.
 11. A process according to claim 1 wherein the copper source comprises CuBr; the ligand comprises valine, proline, or anthranilic acid; the ligand is provided in an amount of two molar equivalents per mole of copper; and the CuBr is combined with the ligand in the presence of water and air.
 12. A process according to claim 1 wherein a base comprises one or more of a water-soluble hydroxide, phosphate, carbonate, or bicarbonate of one or more of Li, Na, K, Mg, or Ca.
 13. A process according to claim 1 wherein a base comprises NaOH or Na₂CO₃.
 14. A process according to claim 1 wherein copper is provided in an amount of between about 0.1 and about 5 mol % based on moles of 2,5-dihaloterephthalic acid.
 15. A process according to claim 1 wherein the ligand is provided in an amount of between about one and about ten molar equivalents per mole of copper. 